1. Field of the Invention
The present invention generally relates to control systems for automotive vehicles. The present invention specifically relates to a control of a brake system of an automotive vehicle for improving vehicle transient and steady state performance in a combined braking and steering maneuver by the vehicle, and an integrated control of a brake and steering system of an automotive vehicle for improving vehicle transient and steady state performance in a combined braking and steering maneuver by the vehicle.
2. Description of the Related Art
Recently many vehicles have been produced with brake systems which can independently control brake forces (i.e., torques) of individual wheels. Many automakers and automotive suppliers are also developing brake by wire systems (e.g., electric or electro-hydraulic) which will give designers more freedom than ever before in controlling braking forces of individual wheels in response to instantaneous conditions of motion as well as access to additional measured signals. At the same time, some vehicles are offered with active rear wheel steer systems, and an intense development efforts continue in the area of augmented front steer or steer by wire systems.
Most efforts in the area of brake control algorithms are focused on improving brake control in an anti-lock braking system (ABS) mode of operation and a vehicle stability enhancement (VSE) mode of operation. These modes of operations are active only when a vehicle is at or very close to the limit of adhesion. During base braking, the brake force distribution typically used is symmetric left to right. Thus, it is not affected by vehicle cornering and is optimized for straight line braking. Therefore, transient response of many vehicles in combined steering and braking maneuvers is less than ideal with a tendency of the vehicle to oversteer and to prematurely enter into ABS mode of operation due to reduced normal loads on the pair of inside tires.
More specifically, FIGS. 1A-1C illustrate the fundamental physical principles of a vehicle 10 in a combined braking and right hand cornering maneuver. As shown in FIG. 1A, vehicle 10 is subjected to a longitudinal force FLO equaling m*ax and a lateral inertial force FLA equaling m*ay, where m is a mass of vehicle 10, ax is a longitudinal acceleration of vehicle 10, and ay is a lateral acceleration of vehicle 10. A pitch and roll moment of vehicle 10 during the maneuver is due to the longitudinal force FLO and the lateral inertial force FLA in combination with various pitch forces PLF, PRF, PLR, and PRR, and various roll forces RLF, RRF, RLR, and RRR, applied to a left front tire 11a, a right front tire 11b, a left rear tire 11c, and a right rear tire 11d, respectively. The pitch and roll moment of vehicle 10 is balanced by various normal forces NLF, NRF, NLR, and NRR being applied to left front tire 11a, right front tire 11b, left rear tire 11c, and right rear tire 11d, respectively. As a result, a normal load distribution among tires 11a-11d is shifted from rear tires 11c and 11d to front tires 11a and 11b due to braking, and from inside tires 11b and 11d to outside tires 11a and 11c due to cornering. Consequently, as shown in FIG. 1B, left front tire 11a carries the largest normal load and right rear tire 11d carries the smallest normal load.
The vectors of forces VLF, VRF, VLR, and VRR in the yaw (horizontal) plane of vehicle 10 developed by each tire 11a-11d, respectively, must remain within a corresponding friction circle 12a-12d, respectively, whose radii are equal to products of a surface coefficient of adhesion xcexc and the corresponding normal force NLF-NRR. If tires 11a-11d are on a relatively uniform surface, the maximum available tire forces in the yaw plane are approximately proportional to normal forces NLF-NRR. With the brake proportioning techniques known in the art, the brake forces on both sides of vehicle 10 are approximately the same. Thus, during braking, the friction potential of outside tires 11a and 11c is underutilized while inside tires 11b and 11d enter ABS too early. In a 3-channel system, an ABS mode is entered on both rear wheels 11c and 11d simultaneously whereby a further reduction in longitudinal forces is generated.
Another undesirable consequence of traditional brake force distribution during a braking and cornering maneuver is that vehicle 10 exhibits a tendency to oversteer, especially under light to moderate braking. FIG. 1C illustrates a simplified bicycle model of vehicle 10 for explaining the aforementioned oversteer condition of vehicle 10. Prior to braking during a steady state cornering, a lateral force Fyfa applied to a front axle (not shown) of vehicle 10 and a lateral force Fyra applied to a rear axle (not shown) of vehicle 10 balance each other whereby a yaw moment MZ about a center of mass 13 of vehicle 10 is approximately zero in accordance with the following equation [1]:
Mz=Fyfa*axe2x88x92Fyra*b=0xe2x80x83xe2x80x83[1]
where a is a longitudinal distance between the front axle and center of mass 13, and b is a longitudinal distance between the rear axle and center of mass 13. Lateral force Fyfa and lateral force Fyra correspond to side slip angles of front tire 11a and rear tire 11c, respectively, with the side slip angle of front tire 11a being larger than the side slip angle of rear tire 11c. 
When brakes are applied to front tire 11a and rear tire 11c, normal force NLF is increased on front tire 11a and normal force NFR is reduced on rear tire 11c. Thus, if the side slip angles of front tire 11a and rear tire 11c were to be maintained, an increase in lateral force Fyfa on the front axle that is nearly proportional to normal force NLF would occur while a decrease in lateral force Fyra on the rear axle that is nearly proportional to normal force NLR would occur. This imbalance between lateral force Fyfa and lateral force Fyra increases yaw moment MZ in accordance with the following equation [2]:
Mz=Fyfa*axe2x88x92Fyra*b greater than 0xe2x80x83xe2x80x83[2]
Consequently, the yaw rate of vehicle 10 increases until a new steady state is reached. Another effect of braking is a reduction of lateral force Fyfa and lateral force Fyra due to development of longitudinal forces (not show). This effect produces an opposite result than illustrated in FIG. 1C, but the effect is significantly small for light and moderate braking, and therefore the first effect dominates. In this new steady state, the side slip angle of rear tire 11c is larger than prior to braking and the slide slip angle of front tire 11a is lower than prior to braking. This is essentially one of the definitions of vehicle oversteer.
There is therefore a need for a brake control method for overcoming the aforementioned shortcomings described herein. The present invention addresses this need.
The present invention provides a novel and unique method and system for improving vehicle transient and steady state performance in a combined braking and steering maneuver by using side to side proportioning of braking forces during braking in a turn. Accordingly, the present invention applies to any brake system that provides means of controlling brake forces among wheels in various proportions (e.g., a hydraulic brake system, an electric brake by wire system, and a hybrid of a hydraulic brake system and an electric brake by wire system). While the present invention is not limited to any particular implementation scenario, the intended area for implementing the present invention is mainly in the range of a performance envelope below the activation of prior art brake control algorithms related to ABS and VSE.
One form of the present invention is a method of dynamically controlling an operation of a vehicle during a combined braking and cornering maneuver by the vehicle. First, a desired brake force for a plurality of tires of the vehicle is determined. Second, a brake force distribution of the desired brake force among the plurality of tires is determined. The brake force distribution is approximately proportional to a normal force distribution among the plurality of tires during the combined braking and cornering maneuver.
A second form of the present invention is vehicle comprising a plurality of tires and a brake controller. The brake controller is operable to determine a desired brake force for the tires during a combined braking and cornering maneuver by the vehicle. The brake controller is further operable to determine a brake force distribution of the desired brake force among the tires with the brake force distribution being approximately proportional to a normal force distribution among the plurality of tires during the combined braking and cornering maneuver.
The foregoing forms, and other forms, features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof.